Light transport simulations are implemented in visual computing and computer graphics applications such as ray tracing, but unfortunately such simulations remain a computationally challenging task. Several approaches have been introduced which convert indirect lighting into direct lighting using many light sources on many shading samples, and apply sophisticated algorithms to reduce the complexity of the resulting many-light problem by exploiting the intrinsic low-rank nature of the resulting transport matrix. Some of these approaches have been described by B. Walter, A. Arbree, K. Bala, and P. Greenberg in the publication “Multidimensional Lightcuts” (SIGGRAPH 2006), and by M. Hasan, F. Pellacini, and K. Bala in the publication “Matrix Row-Column Sampling for the Many Light Problem” (SIGGRAPH 2007). However, due to the unidirectional nature of the indirect to direct conversion process which is essentially based on photon tracing, some phenomena such as sharp glossy reflections and refractions are still very difficult to handle. Moreover, the conversion process introduces weak singularities which need to be removed through clamping, thus introducing bias.
In contrast, bidirectional path tracing, as disclosed by E. Veach in the Ph.D. dissertation entitled “Robust Monte Carlo Methods for Light Transport Simulation,” (Stanford University, December 1997), treats light transport as an integration problem in the space of all possible paths, and provides a Monte Carlo bidirectional path sampling approach which, connecting entire sub-paths traced independently from the light and the eye respectively, can smoothly handle different kinds of transport. Additionally, using Multiple Importance Sampling the algorithm can efficiently remove weak singularities and highly improve the convergence rate without introducing any bias.
The bidirectional path tracing method samples and processes individual transport paths and does not exploit any intrinsic coherence in the underlying light field. To the contrary, as full paths are constructed by connecting vertices of independent sub-paths which have no intrinsic spatial relation, the method prevents a full exploitation of stratified sampling.
What is needed is a method and system operable to estimate light transport which is more efficient and more amenable to applicable sampling techniques.